This invention relates generally to measurements of the radiation properties of an acoustic source, and more particularly to apparatus and methods for holographically reconstructing images of complex acoustic sources.
An important area of acoustics is the study of radiation of sound into a medium (air or water) by a complex vibrator. Research in this area is important in military applications such as tactical quieting of military equipment, especially naval vessels, and passive and active sonar detection of enemy vessels; and the area is important in civil and industrial applications such as environmental noise abatement, and loudspeaker and musical instrument design. This area is also relevant to related fields such as electromagnetic radiation (antenna design) and hydrodynamics (flow-noise research). The fundamental objective in this research area is to correlate properties of the vibrator, such as structural features or vibrational modes, with the properties of the radiated sound, such as the farfield radiation pattern, the source vector intensity field, etc. Because of the complex nature of actual sound sources, the measurement of all the relevant properties using customary techniques is extremely involved (especially in the case of intensity measurements), with the result that thorough and time-efficient measurements are all but impossible.
There have existed a number of measurement methods which provide diverse information about the radiation of sound from sources. These are generally concerned with providing one of the following characteristics of the sound field: farfield directivity, nearfield vector intensity, surface velocity (as for a vibrating plate), and total sound power. To measure the farfield directivity, a microphone in the farfield is passed around a sound source in an anechoic chamber to determine the variation of the sound pressure with angle. These measurements are used in underwater source calibrations, in loudspeaker and musical instrument studies, etc. The disadvantage is that the measurements must be made in the farfield which for large sources or low frequency sources may lie a considerable distance from the source, beyond the size of any available anechoic chamber. There are other techniques which use measurements in the nearfield on cylindrical or spherical surfaces to calculate the farfield directivity. For these measurements a small anechoic chamber is sufficient. The method of the present invention for obtaining the farfield directivity is related to the nearfield techniques and fully retains their advantage.
Measurement of the nearfield vector intensity is currently receiving a great deal of attention in acoustics, and various measurement techniques have been used. U.S. Pat. No. 3,364,461 to W. J. Trott discloses a large planar array of transducers the sensitivities of the individual elements of which are shaded to produce a constant, plane wave near-field extending over the aperture of the array. The shading is such that the sensitivities of the elements increase from the extremeties toward the center of the array according to the coefficients of a summed binomial probability distribution function. While this system affords near-field measurements over a large aperture, and the simultaneous outputs of the elements are integrated to provide some useful response and directivity characteristics of a transducer source, it is subject to the disadvantage of relatively fixed shading values, and is lacking in ability to resolve the individual radiating features of a complex source. In another technique, two closely-spaced microphones are moved in an imaginary surface enclosing a sound source, measuring both the sound pressure and its gradient. From such measurements one component of the vector intensity may be calculated. By scanning over a surface one can determine the average radiated sound power. This technique is limited because coherent measurements can be made only in a small area, and the results to not reveal the complicated nature of the intensity vector field in the vicinity of the source and in the transition region between the source and farfield. There is a further error due to the finite separation of the microphones. A further common intensity measurement technique is similar to the two-microphone technique but uses an accelerometer mounted to the source in place of the second microphone. The accelerometer determines the surface normal component of the pressure gradient and hence only determines the normal component of the intensity. It suffers from the same disadvantages as the two-microphone technique in addition to being time-consuming for large or complicated sources.
The surface velocity or modal pattern is usually determined by mapping the surface with an accelerometer, but like the second intensity measurement technique, this has the disadvantage of being time-consuming. A non-contact method of determining the surface velocity involves optical holography. This technique, however, requires highly specialized equipment and controlled laboratory conditions, and cannot be used to study large areas.
The conventional method of measuring the total radiation sound power is to measure the sound level generated by the source in a reverberant room. The practical disadvantage here is the requirement that the measurements be made in a well characterized reverberant room. Furthermore, the assumptions which must be made about the statistical nature of the room are invalid at low frequencies, which is the regime for many relevant noise sources.
In summary, the determination heretofore of all of the desired properties of a sound source and field using conventional methods has required more than one technique and thus has been involved and time-consuming. Furthermore, each technique suffers from some fundamental limitation.
Now, a large fraction of the important applications for acoustic radiation research mentioned earlier involve low frequency, long wavelength sound radiation, and it is usually assumed in the field of holography that the spatial resolution of a reconstructed image is limited by the wavelength of the radiation. Because of this, acoustical holography has been rejected heretofore as a means of precisely locating and quantifying low frequency sound sources. However, the wavelength resolution limitation is not intrinsic to the fundamental theories of holography but rather is due to experimental limitations which are present in optical holography but are not necessarily present in acoustical holography.